This is a similar problem currently facing a particular sect in Israel. Rather than a gene bank they are allowing brides from "Eastern Europe" only to mate with their men so they do not die out:
The Samaritans, a rapidly dwindling sect dating to biblical times, have opened their insular community to brides imported from eastern Europe in a desperate quest to preserve their ancient culture. Their numbers have been further reduced by the decision by 10 women in recent years to marry outside the community, resulting in excommunication. Today, males outnumber females roughly three to one. With a limited pool of potential partners, it is common for Samaritans to marry within their extended families, even first cousins. As a result, Samaritan babies have suffered from birth defects and genetic diseases at a much higher rate than the general population.
It makes one wonder why they prefer white brides and not brides from elsewhere in the Middle East or Africa since Israel is geographically and culturally closer to them than to Europe.
If such a gene bank were created for Slavs there are a whole host of issues which could arise which are not favorable. For instance, how can you guarantee the authenticity of the samples? One concern about recipients of human sperm and egg from third parties is that plastic surgery and other cosmetic applications can mask inherited flaws in appearance of the donors. Also, some genetic abnormalities skip generations.
In the same context fertility donations can be taken from people currently living in Poland, Ukraine, etc. but the donors could be Tatar, Armenian, Jewish or even some immigrant resident or visiting student from another part of the world. I realize you would expect a better screening process in place beforehand but errors are always made either unintentionally or on purpose.
If you had Slavic donors another question which would arise is what would the world look like if you were to "resurrect" a dying culture? Who would do this resurrecting? Would those resurrected be given guarantees and benefits in society or would they be created for a life of servitude? If they came from a genetic pool of Slavs that is not to say the future of current or historical Slavic countries would still be that way. Some other culture could have come along and replaced them or the lands may one day be uninhabitable. In the article I shared about the Samaritans the women from Russia and Ukraine are expected to abandon their heritage.
The best way to preserve any culture is to ensure broad economic prosperity of citizens along with effective border controls.